1. Field of the Invention
This invention relates to a stencil printer and a method of stencil printing in which the printing matrix area formed on a stencil is determined depending on the size of printing papers.
2. Description of the Related Art
As a stencil printer, there has been known one comprising a cylindrical drum having a porous circumferential wall on which a stencil is mounted, an ink supply section which supplies the inner surface of the circumferential wall with ink, and a pressure roller which is disposed in parallel to the drum at a predetermined distance therefrom. Sheets of paper are supplied between the cylindrical drum and the pressure roller from a sheet feed section when printing.
In the stencil printers of this type, ink is transferred to the sheets through the printing matrix which is formed in the stencil by thermally perforating the stencil. Depending on the relation between the size of the printing matrix area and the size of the printing paper, there arises a problem that the surface of the pressure roller is stained.
Specifically in the case where a stencil having a printing matrix area corresponding to the A3 size is mounted on the drum, when printing is started with sheets of printing paper smaller than the A3 size (e.g., of the A4 size, the B5 size or the like) stacked on a paper feed table, the printing matrix area projects outward beyond the edges of the printing paper and ink passing through the stencil in the projecting portions of the printing matrix area is transferred to the pressure roller to stain the surface of the pressure roller. When printing is continued in that state, the ink on the pressure roller is transferred to the back side of the printing paper, which results in contamination of the printed papers.
Further in the printing drums used in such stencil printers, since ink is impregnated into the porous structure portion of the drum and passes through the printing matrix area of the stencil, each drum is used for one color only, for instance, one for black, one for red, one for blue and so on. Accordingly when the printing color is to be changed, the drum is replaced.
Here it is assumed that a stencil printer A prints on A3 size sheets with a drum for black while a stencil printer B prints on A4 size sheets with a drum for red. Then it is further assumed that necessity of changing the printing color from red to black without changing the printing size (A4) arises in the stencil printer B. In this case, when the operator removes the drum for black which has been used in the stencil printer A and mounts the drum on the printer B in place of the drum for red without taking into the difference in sheet size, black ink is supplied over an area of the drum surface corresponding to the printing matrix area of the A3 size sheets which is larger than the A4 size sheets and the black ink supplied in the portion outside the A4 size sheets is transferred to the pressure roller. A similar problem arises when the printing size is reduced to a size smaller than A3 in the stencil printer A.